1. Field of the Invention
This invention relates to a signal processing apparatus and method used to measure the relative position of a plurality of bodies, and more particularly it relates to such a signal processing apparatus and method suitable for application to a system for aligning a mask and a wafer highly accurately in a semiconductor manufacturing apparatus.
2. Description of the Prior Art
In a semiconductor manufacturing apparatus, especially a pattern printing apparatus, it is the usual practice to photoelectrically detect alignment marks in order to detect the relative position of a mask and a wafer. It is to be understood that the term "marks" used herein refers not only to the indicia added only for alignment, but also to the body itself, for example, a part or the whole of the circuit element, where it is used for alignment.
Where photoelectric detection is to be effected, there is known a method in which a laser, for example, is used as a light source and alignment marks in overlapping relation are scanned and the scattered light therefrom is received by a photodiode or a photomultiplier and photoelectrically converted, whereby the relative position of the alignment marks is found from the electrical signal, also known is a method in which the images of alignment marks are received by an image pickup device such as an image pickup tube or a photosensor array and the entire alignment mark images are photoelectrically converted and input to a signal processing device, whereby the relative position of the alignment marks is found by a calculating process.
However, in the system wherein the marks are beam-spot-scanned along the same track, it has often been difficult to discriminate between the false signals of dust and aluminum particles lying on the body and the true signals regarding the alignment marks. Also, in the system using the image pickup device, it has been necessary to apply complicated signal processing after the entire picture plane has been input, and this has led to a long processing time and thus a slowed printing process or a very high cost or bulkiness of the apparatus. The assignee of the present invention has previously proposed Japanese Patent Application No. 66898/1977 to overcome the difficultes resulting from false signals, but the present invention pertains to improvements different therefrom.